This invention generally relates power amplification circuits. More particularly, it relates to an audio amplifier circuit which reduces power supply noise amplified to the load.
When using a Single-ended (Ground Referenced) technique for power amplification, power supply noise is applied to the load through the output stage. In the case of a power supply that is derived from the AC mains this noise will appear at the power line frequency (50/60 Hz) and its harmonics. When a switching power supply is used to power the amplifier, high frequency noise will appear on the load attached to the amplifier. In an automobile the supply is contaminated by many electrical processes.
There are several methods for removing the above-mentioned noise from the output. One method is to use extremely quiet well-regulated power supplies. When doing power amplification this method is frequently too expensive.
A second method is to use the amplifier to reject the power supply noise. Since high power levels are involved, internalizing power supply rejection can significantly increase the cost of the amplifier. Most integrated power supply rejection techniques drop the available power supply voltage in order to regulate. This means less voltage is available to the amplifier output stage, therefore the amplifier will deliver less power to the load and the power dissipation of the amplifier circuit including the regulator will be increased. (It will generate more heat.)
Other prior art circuits used differential outputs to cancel the power supply noise. However, this creates a differential xe2x80x9cnull pointxe2x80x9d that increased the cost of the amplifier because additional circuitry was required.
The circuit diagram in FIG. 1 shows a prior art method of using the amplifier to reject the power supply noise. The amplifier contains two MOSFET transistors, M1 and M2. They represent the output circuit for a MOSFET power amplifier output stage. The power supply is shown as a DC source (V1) in series with an ac generator (V2) to simulate a power supply with noise. As can be seen from the circuit diagram, the output of the ac generator in the Existing Circuit, V2 will pass through transistor M1 and the capacitor C1, and appear on the load (RL) as a noise voltage.
The present invention introduces a novel technique that involves taking the circuit""s existing output coupling cap and power supply decoupling cap and connecting them in series from the power supply to ground. The output of the amplifier is then directly coupled to the load and the other side of the load is connected at the xe2x80x9cnull pointxe2x80x9d between the two capacitors.
By using this technique, no additional circuitry is required, there is no decrease in available power supply voltage, and the power dissipation of the circuit is not increased.
An advantage of an embodiment of the present invention is a low-cost circuit that will dramatically increase the power supply rejection of the amplifier circuit. Another advantage of and embodiments is it reduces the capacitor count and size in a stereo amplifier while simultaneously eliminating the normal fall in low frequency response. Further advantages of embodiments include not decreasing the available power supply voltage, or increasing the power dissipation of the amplifier including the regulator.
An embodiment of the present invention includes a method of improving the low frequency response of an ac-coupled amplifier without a proportional increase in output coupling capacitance.